Vehicular pollutant concentrations within approximately 300 meters (approximately 984 ft.) of major highways frequently exceed minimum air quality criteria recommended or specified by government agencies (e.g., standards or guidelines). As is well known, vehicular emissions include pollutants such as carbon monoxide, oxides of nitrogen, particulate matter, and volatile organic compounds that are known to have human health effects. In addition to emissions from vehicular tailpipes, airborne pollutants emanate from the roadway surface, e.g., due to tire wear. For the purposes hereof, all airborne vehicular emissions and airborne pollutants emanating from the roadway surface are collectively referred to as “roadway pollutants”.
In the prior art, mitigation (i.e., improvement) of air quality in the regions proximal to a busy roadway is generally not easily achievable. For example, a typical approach to mitigating air quality in the vicinity of a roadway is to purchase or expropriate land along the roadway, to provide larger regions (i.e., on both sides of the roadway) in which the roadway pollutants may disperse. In effect, this approach involves providing wider leeward regions along the roadway from which residences and businesses have been removed, e.g., via expropriation. The polluted air (i.e., air polluted by the roadway pollutants) from a region (the “roadway region”) generally above the roadway is therefore required to travel further before reaching residences or businesses, and accordingly, the roadway pollutants are more likely to have been dispersed in the leeward region, i.e., before the polluted air from the roadway region reaches residences or businesses located adjacent thereto. However, this approach is impractical along many roadways, especially due to existing land uses proximal to older roadways. Even where this approach is not impractical, it is extremely expensive, and the process of obtaining the land may take several years.
Typically, costly and time-consuming environmental assessment proceedings are required to be successfully completed before construction of major new roadways (or major expansions of existing roadways, as the case may be) may proceed. Often, concerns about air quality in regions leeward to the roadway result in significant delays in an environmental assessment. However, as noted above, in the prior art, mitigation of air quality proximal to the roadway requires government acquisition of larger (leeward) areas of land along the roadway, i.e., where this is not impractical.
The prior art is schematically illustrated in FIGS. 1A, 1B, and 2. FIG. 1A shows the flow of polluted air from the roadway region toward the leeward region in the absence of any obstruction (e.g., a wall) mechanically affecting such flow of polluted air. As can be seen in FIG. 1A, in the absence of such an obstruction, the polluted air typically flows substantially parallel to the ground.
The impact of two different prior art walls on the flow of air from the roadway region is shown in FIGS. 1B and 2 respectively. The air flows schematically represented in FIGS. 1B and 2 were determined via computer-generated modelling using computational fluid dynamics software.
Most tailpipe emissions from automobiles and light trucks are released at a certain height (e.g., about 0.5 meters above the roadway surface), and most tailpipe emissions from larger vehicles (e.g., trucks, buses) are released at a height of about 3 meters above the roadway surface. In addition, and as described above, certain airborne pollutants emanate from the roadway surface, and these roadway surface-related pollutants also are generally considered, for modelling purposes, to be released at about 0.5 meters above the roadway surface. Accordingly, for modelling purposes, the polluted air can be considered to have two sources, namely, a lower source (designated “OL” in FIG. 1A) from which tailpipe emissions from automobiles and light trucks, and airborne pollutants emanating from the roadway surface are considered to be emitted, and an upper source (designated as “OU” in FIG. 1A) from which tailpipe emissions from larger vehicles are considered to be emitted.
At different heights above the roadway, the air in the roadway region has been found to include different concentrations of roadway pollutants therein, with higher concentrations in lower regions and concentrations generally decreasing as height above the roadway increases. The polluted air (and the less polluted air) in the roadway region may be considered to be roughly divided into layers based on different concentrations of roadway pollutants. For example, in a typical roadway region, in a lower band identified in FIG. 1A (from the roadway surface to approximately two meters above the roadway), the concentration of pollutants typically may be about 50 μg/m3. In a middle band, from about two meters to about four meters above the roadway, the concentration typically may be about 40 μg/m3. Finally, in an upper band extending between about four meters and about six meters above the roadway, the concentration typically may be about 5 μg/m3.
In the prior art, walls are often located beside busier roadways, in an attempt to address noise concerns. Such prior art walls are generally indicated by the reference numerals 10 and 12 in FIGS. 1B and 2 respectively. Such prior art walls are intended to reduce noise effects, i.e., to mitigate the extent to which noise from traffic on the roadway affects those living or working in regions leeward to the roadway.
However, modelling shows that the prior art walls 10, 12 affect the flow of polluted air from the roadway toward the leeward region. For example, based on such modelling, FIG. 1B schematically illustrates the effect which the prior art wall 10 has on polluted air flowing from a roadway region 11 to a leeward region 13. The wall 10 is a standard noise wall of the prior art.
For modelling purposes, as described above, the polluted air is considered to originate from two sources in the roadway region 11. These two sources are the lower source OL and the upper source OU, described above. The lower source is assumed to be positioned about 0.5 meters above the ground surface 14. The lower source is intended to represent exhaust gases (and particulates) from automobile tailpipes and re-entrained roadway emissions, e.g., due to particulate matter on the roadway surface. For the purposes hereof, the emissions from automobile tailpipes and the roadway emissions are collectively referred to as “lower source pollutants”. The upper source is positioned about 3 meters above the ground surface 14. The upper source is intended to represent exhaust from a truck or a bus from which exhaust gases (and particulates) are released at about 3 meters above the ground, and such pollutants are collectively referred to as “upper source pollutants”.
In general, the larger particulates (i.e., TSP (total suspended particulates, meaning those sized less than about 44 μm.) and PM10 (particulate matter sized less than about 10 μm.)) settle on the roadway side of the prior art wall 10. However, smaller particulates (i.e., PM2.5 (particulate matter sized less than about 2.5 μm.)) tend to be carried over the wall 10, to settle on the leeward area.
As can be seen in FIG. 1B, much of the air flow from the lower source OL is blocked by the wall 10, resulting in relatively high positive pressure on the windward side of the wall 10, i.e., at 16. At the same time, air flow over the top of the wall 10 from the upper source OU tends to be somewhat separated at the leeward side of the wall (i.e., at 18) near the top of the wall, due to the wake effect. Turbulence (generally, at 20) results from the wake effect. This permits airborne contaminants (i.e., the particulate matter generally sized less than about 2.5 μm.) to mix and eventually to settle on the ground surface generally leeward of the wall 10.
As can be seen in FIG. 1B, the flow of air from the roadway region 11 which is at a height “H1” above the noise wall 10 is undisrupted, i.e., where the wall-induced separation (indicated at 18) and turbulence (indicated at 20) can not reach to the height H1.
Based on the modelling, and as schematically illustrated in FIG. 1B, the net effects of the wall 10 on movement of polluted air from the roadway are summarized as follows.                (a) The blocking effect of the wall 10 on air from the lower source tends to reduce concentrations, at a receptor “R1”, of the lower source pollutants.        (b) The separation of air flows and turbulence (at 18 and 20 in FIG. 1B) tends to increase concentrations of upper source pollutants at the receptor R1.        
The receptor R1 is considered to be located at about 1.5 meters above ground level. This height for the receptor was selected because it is a height at which, in general, human beings inhale. It is therefore considered to be an appropriate location at which to measure a person's exposure to airborne roadway pollutants.
In summary, the prior art wall 10 results in somewhat higher concentrations of upper source pollutants in the leeward area. Based on the modelling, it appears that these higher concentrations are found in the leeward area within approximately 110 meters of the wall 10.
The concentration of the upper source pollutants in the leeward region is generally undesirable. It is particularly serious, however, in circumstances where large trucks and/or buses typically are collected on a part of a roadway, and segregated from other vehicles. These circumstances may occur, for example, where roadways cross international borders.
As can be seen in FIG. 2, the prior art wall 12 is substantially the same as wall 10, but with an angled part 22 positioned upon it. In FIG. 2, much of the air flow from the lower source OL is blocked by the wall 12, which also results in relatively high positive pressure on the windward side of the wall 12, i.e., at 24. At the same time, air flow over the top of the wall 12 from the upper source OU is significantly impeded by the angled part 22. However, as indicated in FIG. 2, part of the air flow from the upper source OU flows over the angled part 22. The air which flows over the angled part 22 tends to be somewhat separated at the leeward side of the wall (i.e., at 26) near the angled part 22, due to the wake effect. A negative pressure zone is created, and it appears that such negative pressure zone causes somewhat more turbulence mixing leeward of the wall 12 than had resulted from the straight wall 10, illustrated in FIG. 1B. Also, wind flow turbulence mixes down within the leeside wake (cavity) zone, allowing airborne contaminants (i.e., the particulate matter generally sized less than about 2.5 μm.) to settle in the region of the leeward area designated as 28.
As can be seen in FIG. 2, the flow of air from the roadway region which is at the height “H1” above the noise wall 12 is slightly disrupted, due to stronger separation 26 and turbulence 28 by part 22.
Based on the modelling, and as schematically illustrated in FIG. 2, the net effects of the wall 12 on movement of polluted air from the roadway are summarized as follows.                (a) The blocking effect of the wall 12 on polluted air from the lower source OL (at 24) tends to reduce concentrations, at a receptor “R2”, of the lower source pollutants.        (b) The separation of air flows and turbulence (at 26 and 28 in FIG. 2) tends to increase concentrations of upper source pollutants at the receptor R2.        
As can be seen in FIG. 2, the receptor R2 is considered to be located at about 1.5 meters above ground level.
In summary, the prior art wall 12 appears to have unintended results similar to the unintended results of the prior art wall 10 described above. In particular, the prior art wall 12 appears to result in somewhat higher concentrations of upper source pollutants in the leeward area. Based on modelling, it appears that these higher concentrations are found in the leeward area within approximately 90 meters of the wall 12.